Generally, injection molding of a resin product is carried out through a molding cycle comprising a resin melting process, a filling process, a holding process, and a cooling process. In order to obtain a molded product having an excellent quality, it is a common practice to control a temperature of a mold comprising a fixed mold and a movable mold, a temperature of a resin material within the mold, an injection pressure, and the like. In addition to such control, it is also important to control a clamping pressure applied to the mold. Furthermore, it is important to determine a timing of switching from the filling process to the holding process.
FIG. 1 schematically shows an example of injection molding machine of a horizontal type where the mold is clamped in a horizontal direction. The injection molding machine is basically comprised of an injection unit 10 and a clamping unit 20. In the injection unit 10, a resin material is put into a hopper 11 and are supplied to a heating cylinder 12. The resin material is melted in the heating cylinder 12 while being kneaded and measured by a screw 13. The melted resin material is reserved in a forward area before the screw 13. The screw 13 is rotated by a rotation driving mechanism (not shown). The screw 13 is also driven by a first hydraulic cylinder mechanism comprising an injection cylinder 14 and a piston 15 and is moved forward, namely, towards a mold. When the first hydraulic mechanism makes the screw 13 move towards the mold, the melted resin material reserved in the forward area before the screw 13 is delivered through a nozzle 16 to be filled within a cavity of the mold comprising a fixed mold 17 and a movable mold 18. In a filling process and a holding process, driving oil having a controlled flow speed and pressure flows into and flows out from an injection cylinder 14 through an inlet/outlet port 14-1.
The clamping unit 20 includes a fixed platen 21 to which the fixed mold 17 is attached and a rear platen 22. Both platens 21 and 22 are fixedly supported through four tie bars 23 (only two of them are illustrated). Behind the rear platen 22, a second hydraulic cylinder mechanism including a hydraulic cylinder 24 and a piston 25 is formed. The piston 25 is coupled to a movable platen 26 to which the movable mold 18 is attached. The movable platen 26 is slidable along the tie bars 23 with the movement of the piston 25. Thus, when the driving oil is fed from an inlet/outlet port 24-1 to the hydraulic cylinder 24, the movable platen 26 is moved in a direction such that the mold is closed. At this time, the driving oil flows out from an inlet/outlet port 24-2. On the other hand, when the driving oil is fed from the inlet/outlet port 24-2 to the hydraulic cylinder 24, the movable platen 26 is moved in another direction such that the mold is opened. At this time, the driving oil flows out from the inlet/outlet port 24-1.
The hydraulic cylinder 24 is provided with a pressure sensor 27 for detecting a hydraulic pressure. While the mold is closed, the pressure sensor 27 detects a hydraulic pressure within the hydraulic cylinder 24. The clamping pressure is controlled in response to the pressure detection signal from the pressure sensor 27. The fixed platen 21 and the movable platen 26 are provided with a distance sensor 28 for detecting a platen distance. After the predetermined time length of the holding process and the cooling process, the movable mold 18 is retracted and a molding article (resin material) is retrieved from the mold.
FIGS. 2A and 2B schematically show another example of injection molding machine of a vertical type where the mold is clamped in a vertical direction. FIG. 2A is a front view and FIG. 2B is a plan view, respectively, of the injection molding machine. In an injection unit 30, a resin material is put into a hopper 31 and is supplied to a heating cylinder 32. The resin material is melted in the heating cylinder 32 and is kneaded by a screw 33. The screw 33 is driven by a hydraulic cylinder mechanism 34 and is moved towards the mold. The melted resin material reserved in the forward area of the screw 13 is delivered through a nozzle to be filled within a cavity of the mold comprising a lower mold 37 and an upper mold 38.
A clamping unit 40 has a fixed platen 41 on which a transfer table having the lower mold 37 is placed and an upper platen 42 which is fixedly supported through four support bars 43 (only two of them are illustrated). On the upper platen 42, a second hydraulic cylinder mechanism 44 is formed. The piston 45 is arranged in the hydraulic cylinder mechanism 44 and is coupled to a movable platen 46 to which the upper mold 38 is attached. The movable platen 46 is slidable along the support bars 43 with the movement of the piston 45. Thus, through the hydraulic pressure, the movable platen 46 is moved in a direction such that the mold is closed or opened. When the mold is closed, the melted resin material from the injection unit 30 is filled in the cavity of the mold to form the molding article.
After the predetermined time length of the holding process and the cooling process, the upper mold 38 is retracted. The lower mold 37 is withdrawn from the clamping unit by, for example, rotating the table 49, for removing the molding article (molded product) from the mold. In this example, the table 49 has two lower molds 37 so that while one lower mold 37 is in the cooling process and article removing process at the outside of the clamping unit 40, the other lower mold 37 can be clamped for the filling process and holding process in the clamping unit 40.
To improve the production efficiency, a plurality of molds can be rotatably provided to the clamping unit in a rotary type injection molding machine. FIG. 3 is a schematic plan view showing an example of rotary type injection molding machine. Similar to the above examples, an injection unit 50 includes a hopper 51, a heating cylinder 52, a screw 53, and a hydraulic cylinder mechanism 54. A clamping unit 60 has a structure substantially the same as that shown in FIG. 2A. In this example, a table 69 has four lower molds 57 thereon. By rotating the table 69, the lower molds 57 are sequentially supplied to the clamping unit 60 for the filling and holding processes. Since the cooling process which requires a relatively long time can be done outside of the clamping unit 60, it is possible to improve the production efficiency.
The foregoing conventional technologies are insufficient for further improving the production efficiency. For example, in the rotary type injection molding machine of FIG. 3, since the order of the lower molds 57 to be supplied to the clamping unit 60 is fixed because of the rotation direction of the table 69, once a problem arises, for example, in one of the lower molds 57, a total system has to be stopped. Further, during the holding process and at least a part of the cooling process, the lower mold 57 has to be stayed in the clamping unit 60 so that the mold is closed during that periods. Therefore, an injection molding system designed by a totally new concept is desired for dramatically improving the production efficiency.